Datum Feature Simulator of Complex Shape

[Latency]

Made the graphics a 2d part for simplicity.

I would like a better understanding of the what the datum feature simulator would be for the comlex boundary shape.

Please see attached image.

Is this complex boundary a valid datum feature A which can be used to consistent define datum feature simulator?

Which surfaces of feature A would be use to make this datum simulator for the purposes of validating location of holes with respect to A.

 

[Latency]

File attached.

 

[MechNorth]

Unless this is something like a key, which will be completely encompassed in a relief, it's a poor choice of datum. You would have to literally contact every surface of the outer perimeter to simulate it, and it wouldn't add any value for the significant complexity. Figure out how this workpiece mates to the other components, and post that information; then we can offer practical help.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DanStro]

I have to agree with MechNorth. There aren't too many other features shown but I think that either the large single hole or the hole pattern would be a better choice for the datum, depending on the function of the part. with the profile with respect to it.

Han primo incensus

 

[Latency]

The real part has many clearance holes (20x +) in it. No hole in particular is important. The part itself is a spacer/shim piece made of compliant/rubbery material. No specific part of the outline is important (composed of many steps and radius). The line profile tolerance would be fairly loose of .040 all around.

The part itself is very thin < 0.032 so I did not want to hold any 3d tolerances due inspection difficulty and no value added to function.

Initially I was thinking of using the pattern of holes as datum A and hold the line profile of the outer boundary to it. But asme gdt spec has ambiguous definition of how datum would be established with pattern of holes. Due to flimsiness of piece, I doubt an inspection fixture would be practical way to inspect it.

I gave it some more thought, perhaps making the overall width datum A and overall length datum B might make sense. Just to establish something to relate the pattern of hole to bounding outline.

 

[MechNorth]

If there are no critical relationships overall, then consider using a single datum (the back face as Datum feature A), then control everything with a general surface profile tolerance. A part like this (thin & flexible) is typically checked by vision system or manual optical comparator. Using a general profile tolerance gives all featues a "zone" or set of inner and outer boundaries within which each of the features must reside. Picture a mylar transparency of the nominal geometries with inner & outer boundaries for each feature being shown. You put the mylar up against the projection of the workpiece on the optical comparator, and wiggle the mylar around until you get all the features to fit within their designated boundaries simultaneously.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com